Dehesa, San Diego Co., California, USA © Caltech

Chemical Formula: (Al,Fe3+)7(SiO4)3(BO3)O3
Locality: San Diego Co., California.
Name Origin: Named after the French paleontologist, M. E. Dumortier (1803-1873).

Dumortierite is a fibrous variably colored aluminium boro-silicate mineral, (Al,Fe3+)7(SiO4)3(BO3)O3. Dumortierite crystallizes in the orthorhombic system typically forming fibrous aggregates of slender prismatic crystals. The crystals are vitreous and vary in color from brown, blue, and green to more rare violet and pink. Substitution of iron and other tri-valent elements for aluminium result in the color variations. It has a Mohs hardness of 7 and a specific gravity of 3.3 to 3.4. Crystals show pleochroism from red to blue to violet. Dumortierite quartz is blue colored quartz containing abundant dumortierite inclusions.

Dumortierite was first described in 1881 for an occurrence in Chaponost, in the Rhône-Alps of France and named for the French paleontologist Eugène Dumortier (1803–1873). It typically occurs in high temperature aluminium rich regional metamorphic rocks, those resulting from contact metamorphism and also in boron rich pegmatites. The most extensive investigation on dumortierite was done on samples from the high grade metamorphic Gfohl unit in Austria by Fuchs et al. (2005).

It is used in the manufacture of high grade porcelain. It is sometimes mistaken for sodalite and has been used as imitation lapis lazuli.

Sources of Dumortierite include Austria, Brazil, Canada, France, Italy, Madagascar, Namibia, Nevada, Norway, Poland, Russia and Sri Lanka.

Physical Properties

Cleavage: {100} Good, {110} Indistinct
Color: Blue, Brown, Violet, Greenish blue, Pink.
Density: 3.3 – 3.4, Average = 3.34
Diaphaneity: Transparent to translucent
Fracture: Fibrous – Thin, elongated fractures produced by crystal forms or intersecting cleavages (e.g. asbestos).
Hardness: 8.5 – Chrysoberyl
Luminescence: Non-fluorescent.
Luster: Vitreous (Glassy)
Magnetism: Nonmagnetic
Streak: white

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Photo :

This sample of Dumortierite is displayed in the Smithsonian Museum of Natural History. This sample is about 15×20 cm and is from Dehesa, California.
Locality: Botswana FOV: 1.5 cm. © Val Collins